<template>
  <div>
    <ul>
      <li v-for="(item, index) in arr" :key="index" v-html="item"></li>
      <button @click="btn">new</button>
    </ul>
  </div>
</template>

<script>
export default {
  data() {
    return {
      arr: ["老大", "老二", "老三"],
    };
  },
  methods: {
    btn() {
      this.$set(this.arr.splice(1,0,'new'))
    },
  },
};
</script>

<style>
</style>